I was told here (in korea) that it's because the e510 has better build quality - but since the guy only had an e510 in stock, it's perhaps a lie.

I think the seller was definitely lying. The E-520 and E-510 have nearly the exact same body. The only real differences in the build are the mode dial, screen size (2.5 vs 2.7), color of certain symbols, and the name on the front.